As the topic says some sort of blacklisting for diamond pipes would be nice as well as a possibility to describe "nothing goes here" or "everything goes here" (instead of using like an enchanted pickaxe as item for "nothing goes here")
also there currently is no compact solution for saying "half the wood and everything else to the yellow side" and "the other half of wood to the red side"
This might could be achieved by giving the list item a "blacklist" option (thus empty blacklist means everything goes here) + some finetuning to handle items that explicitely shall go somewhere else partially

For the "accept items" in a gate it would be nice to have blacklists in order to describe that situation: "wood here, saplings there, everything else goes towards somewhere else" especially the "everything else" is currently not possible thus leading to the case that once the robot goes to the docking station that accepts anything it also drops wood and saplings.

Just my opinion

• Nothing goes here is a good idea.
• Everything goes here is called an unfiltered side.
• Blacklisting is rushing a bit into feature creep for me, as you can already do it - just invert your whitelist.
• Adding your proposed compact solution for "half the wood and everything else" etc. would probably cause a lot of quirky edge cases which increase the amount of things you need to remember/keep in mind to use BuildCraft pipes. That would be far worse than a small reduction in compactness, and compactness was never an explicit goal.

As for "accept items", a priority system might be cool.

How do i invert a whitelist? (or anything at all - some sort of "not" would be nice for gates aswell because otherwise you must use a redstone output negate with vanilla logic and redstone input into gates)

You make one side of a diamond pipe the whitelist, then the other side is "everything but the whitelist". NOTs are tough, as we don't want to make the mod too trivial. Risks everywhere..

Maybe even consider "nothing goes here" being the default state and in order to disable it you must click on the corresponding field.
An even more complex (but in my opinion way better) solution would be: when everything is on "nothing goes here" (default state when placing) then items just keep their movement direction because that'd allow crossing two different pipes aswell as building a sorting system w/o having to set every single field to "nothing" or "all" (not "nothing") but instead you'd set the fields only if you need them (because you know what you wanna put in a given chest)

These wouldn't be too trivial since that all works right now already. In case of the "default setting" it's just for convenience and in case of the "straight" mode for the diamond pipes the current way to build such sort of functionality is to use a few extra pipes (not that expensive). But if you wanna use a straight connection you pay for it (with diamonds) so it still should be balanced.
And once you come to endgame where you have lots of resources anyway it'd allow you to tweak your factories even more :)

@asiekierka what's your opinion about it? (In case my english prevents you from understanding what i describe just ask, I'd be glad to clarify what I mean)
:)